1. Field of the Invention
The present invention relates to a process for producing shaped bodies having thermally insulating properties and based on foamed alkali metal silicate particles comprising silicon dioxide (SiO.sub.2) and alkali metal oxide (M.sub.2 O) (M is an alkali metal) in a molar ratio of from 2 to 4.5 (SiO.sub.2 :M.sub.2 O). The invention further relates to the use of the shaped bodies produced according to the process.
2. The Prior Art
Japanese published specification JP-49069755 describes a process for producing shaped bodies based on foamed alkali metal silicate particles. In a first process step, granular material of water-containing sodium silicate is heated and thus foamed. The foamed particles are subsequently sprayed with water or an aqueous sodium silicate solution, placed in a mold and fired to give shaped bodies.
German Patent DE-3246619 describes foamable and curable molding compositions of SiO.sub.2, further oxides, water and foaming agents such as perborate, and processes for the production thereof. According to German Auslegeschrift DE-537492, silicate foam beads are mixed with water glass, an acid compound controlling the curing of the water glass, such as sodium fluorosilicate or phosphoric acid, and a hydrophobic agent, such as polyethylhydrosiloxane, and are solidified to give shaped bodies.
A disadvantage of the shaped bodies based on foamed alkali metal silicate particles which are produced by the known processes is their insufficient dimensional stability at high atmospheric humidity. In particular, at relatively high temperatures and high atmospheric humidity, they lose their original strength. This leads to the bonding between foamed alkali metal silicate particles being mostly lost after prolonged storage under such conditions.